Specs for Float#next_float and #prev_float (Ruby 2.2+)

New in MRI `2.2.0-preview1`.

Discussion was here:
https://bugs.ruby-lang.org/issues/9834

MRI commit here:
https://github.com/ruby/ruby/commit/63fee735002b34d37598d4830ef35073202dda58
Heh.  Good call.  I forgot that `Float#==` != `Float#eql?`.